In microelectronics industry Ge wafers are important substrates with technological applications in optical devices and are very recently introduced as a replacement for Si substrates for advanced Integrated Circuit (IC) devices. In order to realize high performance devices the Ge wafer surface has to be of high purity.
Earlier studies have demonstrated the detrimental effects of metallic contaminants on the electro-physical properties of the Ge surface. Therefore, it is needed to control carefully the metallic contamination during processing of Ge wafers. Specifications for metallic contamination are set to levels below 5E9 at/cm2.
Cleaning recipes have to be developed and performances evaluated by an appropriate analysis methodology. Whereas several studies focused on the optimization of cleaning recipes, there is still a lack in the metallic contamination analysis for Ge wafers.
Direct-Total Reflection X-Ray Fluorescence (D-TXRF) is unrivalled for the direct contamination analysis on Si wafers. The Detection Limits (DL) for this technique have been evaluated also for Ge wafers and are in the order of E10-E11 at/cm2. To meet the more stringent requirements of the IC processing environment further development of the metrology is hence needed.
For the analysis of metallic contamination on Si wafers, the combination of the pre-concentration method of Vapor Phase Decomposition-Droplet Collection (VPD-DC) with micro-volume analytical techniques is a well-established method, which is patented by Maeda et al. (U.S. Pat. No. 4,990,459). The method consists of different subsequent steps. During the VPD step, the native oxide of the Si wafer is etched by an HF fume resulting in a hydrophobic Si surface. In the subsequent DC step the wafer surface is scanned with a micro-droplet of an aqueous mixture to collect the metallic impurities. This droplet can then be analyzed with any wet chemical micro trace analytical technique such as Graphite Furnace-Atomic Absorption Spectrometry (GF-AAS) or Inductive Coupled Plasma-Mass Spectrometry (ICP-MS). Alternatively, the micro-droplet is dried on a carrier substrate and the resulted residue is analyzed by TXRF. The combination of VPD-DC and TXRF for Si wafers is studied in detail by C. Neumann and P. Eichinger (Spectrochim. Acta 46B, p1369, 1991) and presented by D. Hellin et al. (‘Validation of VPD-DC for metallic contamination analysis of Si wafers’, TXRF2003 Conference, Hyogo, Japan). This technique, however, does not work in case of Ge wafers, because an aqueous mixture is not phobic to Ge and droplet loss occurs during scanning.
For metallic contaminants on small size Ge wafers (100 mm), a pre-concentration method is based on the Droplet Sandwich Etch method (DSE), described by D. Hellin, et al. (‘Determination of metallic contaminants on Ge wafers using Direct- and DSE-TXRF spectrometry’, In press, Spectrochim. Acta part B (2003)). In this method, a droplet of a chemical mixture is deposited on a clean carrier substrate. The substrate of interest is then placed on the carrier substrate with the side to be analyzed towards the carrier, sandwiching the droplet. Upon removal of the top substrate a part of the chemical mixture remains on the carrier substrate. This liquid can then be analyzed by any micro-volume analytical technique. However, this methodology suffers from severe limitations with respect to automation and scalability to large wafer sizes.
The VPD-DC methodology as described by Maeda et al. requires the surface of the object to be measured to be hydrophobic to allow scanning by an aqueous solution. If the surface of the object is hydrophilic it has to be rendered hydrophobic by a vapor phase treatment.
Since Ge substrates are not hydrophobic, both HCl and HF vapor treatments are tested to render the Ge surface hydrophobic.
This treatment resulted in rather hydrophilic surfaces. The contact angle after the treatments measured by dispensing a micro-droplet of water (in essence) onto the treated surface resulted in values of about 15 deg. Consequently, the Ge surface could not be scanned by the water droplet as it wetted the surface and split into multiple smaller droplets when scanning. In addition, the treated surface cannot be scanned with one of the solutions Maeda proposed: HF, HF+HNO3, HF+H2O2 and HCl+H2O2.
Solutions of HF up to 49 wt. % were tested.
After application of the VPD step, the Ge surface was not phobic to any of these solutions and droplet loss occurred during scanning.